You can cut the fin base shorter than the actual base of the fin surface, so no need to let that factor influence the choice. IN fact the standard Drake Twin fin delivered with the Quads has a super wide shape but still a 10cm slotbox base.
Also note that you need to route out a recess for the screws to fit if you buy a us box fin.
Regarding fin choice, I for exemple think the MFC Levi Siver twin model would work really well with this board. Size depends on your weight, sail sizes and preferences.